1. Field of the Invention
The present invention relates to a resistance welding control apparatus for carrying out inverter-controlled AC resistance welding.
2. Description of the Related Arts
In an inverter-controlled AC resistance welding, the primary coil of a welding transformer is connected to the output terminal of an inverter, while the secondary coil thereof is connected directly (without interposition of a rectifying circuit) to a pair of welding electrodes. The inverter comprises positive side switching elements and negative side switching elements, of which polarities correspond to current polarities. The inverter receives a DC power, acquired by rectifying a commercial frequency, from a rectifying circuit and is controlled by an inverter control circuit.
For each weld period T.sub.A corresponding to half cycle T.sub.W /2 of a cycle T.sub.W which is defined for the secondary alternating welding current, the inverter control circuit alternately provides a switching control, with a high frequency, to the positive side switching elements and negative side switching elements of the inverter. More specifically, during the weld period T.sub.A corresponding to the positive half cycle of the alternating welding current, the positive side switching elements are switching with a high frequency, e.g., 10 kHz, with the negative side switching elements remaining off, whereas during the weld period T.sub.A corresponding to the negative half cycle of the alternating welding current, the negative side switching elements are switching with the same high frequency (10 kHz), with the positive switching elements remaining off.
Thus, as shown in FIGS. 5A and 5B, the primary coil of the welding transformer is supplied via the output terminal of the inverter with a high frequency pulse whose polarity is inverted at every weld period T.sub.A, while in the secondary circuit of the welding transformer the alternating welding current with a frequency T.sub.W flows through a pair of welding electrodes into materials to be welded, subjecting the welding areas of the materials to the resistance welding.
Such an inverter-controlled AC resistance welding makes use of a low frequency corresponding to the commercial frequency in order to switch the polarity of the welding current, so that it is applicable to the welding head (including the welding transformer and secondary circuit) common to the ordinary low frequency AC of the thyristor-controlled system. In addition, the ratio of the effective weld time is remarkably larger than the ratio of the non-weld time at all times as compared with the thyristor-controlled system, thereby ensuring a stable resistance welding with a higher heat generation efficiency and less spatter.
Nevertheless, the inverter-controlled AC welding poses a problem that the supply of the welding energy is temporarily interrupted upon the polarity switching as shown in FIG. 6, which may affect the weld quality. To obtain a high weld quality, it is demanded to minimize the interruption (dip) of the welding energy upon the polarity switching to thereby enhance the heat generation efficiency and reduce the weld time.
In this respect, the conventional resistance welding control apparatus of this type employs a high-frequency switching of the switching elements on the associated polarity side from the beginning of each weld period T.sub.A to the end thereof. It is however difficult for such a switching control to minimize the interruption (dip) of the welding energy since it takes time to build up the current to the preset value immediately after the beginning of each weld period.
In the case of the resistance welding allowing a flow of large welding current (e.g., 8000 A or more) in particular, the current build-up characteristics will have a significant effect on the heat generation efficiency of the entire welding current supply, since the current supplying cycle count, i.e., the number of weld periods is large with the build-up of the current being iterated many times.